"Smart" COTS multifunction I/O designs revolutionize data distribution systemsStory
October 06, 2010
As modern and future platforms increasingly rely on electronics for control, weapons management, and sensor data integration, the number of sensors and control points used in these platforms has expanded exponentially. Coinciding with this expansion is a significant increase in the requirements for handling this sensor data, while real-estate savings needs and redundancy requirements increase.
However, smart distributed I/O boards and Sensor Interface Units (SIUs), when implemented prudently, can satisfy the demands of these conflicting requirements.
Modern military platforms increasingly rely on electronics for operation. Just as aircraft have transitioned to fly-by-wire systems, ground vehicles are transitioning to drive-by-wire. Simultaneously, greater numbers of sensors are being incorporated into sea, air, and ground vehicles, and these sensors are used to detect everything from incoming projectiles to engine vibration. Coinciding with this are increasing numbers of electronically controlled actuators to affect a response based on the sensor data. Each of these sensors and actuators must be connected to some type of I/O point to collect data for processing. As an example, consider a simple turn-by-wire design. The operator (driver) turns right, requiring an input measurement. Then a control point is used to activate the correct actuator to change wheel position around the vertical axis.
So the question is: How do we manage the increased electronic content of all these sensors without increasing size or weight, then process this exponential increase in data? As always, using more electronics implies higher processing requirements and an increased number of interconnects, which raises even more issues. The answer to this paradox lies in modern “smart” distributed I/O boards and Sensor Interface Units (SIUs), which can satisfy the need for:
- High levels of analog integration
- Sophisticated processing within modern bandwidth limits
- Meeting tightening real-estate requirements
- Easing I/O data and processing distribution
- Mission-critical redundancy
High analog integration levels: “Smart” I/O helps
To accommodate this sensor-induced, increased demand on electronics, using high levels of analog integration proves a viable remedy: two to three times the number of I/O points can be placed on a single PCB today than in the recent past. “Smart” I/O boards help increase analog density by performing many functions previously performed by ancillary analog circuits digitally, such as calibration, filtering, and BIT. Although certainly not proceeding at the same pace as digital electronics integration, nonetheless, analog integration is proceeding along a rapid slope. Whereas five years ago, 32 A/Ds or D/As on a single PCB was considered high density, today 60 or more is commonplace. In fact, it is now to the point that the number of analog functions that can be placed on a PCB is limited more by the number of I/O pins on the connectors than by the density of the electronics that can fit in a given PCB real estate. To address this problem, manufacturers are turning from VME or CompactPCI to VPX (VITA 46). VPX has approximately twice the I/O of VME.
“Smart” I/O boards: Sophisticated processing
The I/O boards being provided today are “smart.” These boards not only perform A/D, D/A, or simple I/O functions, but they also provide sophisticated data-processing levels. For example, simple I/O boards have A/D converters that sample the input data at rates from thousands to millions of samples per second, then send that data to a mission computer for processing. Modern military ships, submarines, aircraft, and even ground vehicles can have many thousands of points to sample. For example, think of the thousands of vibration sensors needed on a modern submarine to ensure complete stealth.
Additionally, in the desire to gain more information over ever-rising bandwidths, the sampling rate is increasing from a few thousand Hz to more than 100 KHz, with bit depths increasing from 12 bits to 16 and 24. These data rates can easily overwhelm the most powerful processor, especially when complex algorithms like FFTs and FIR filtering need to be performed. With onboard FPGAs and DSPs, I/O board manufacturers can easily provide preprocessing to offload the main processor. Since these components are highly programmable, the hardware of these COTS boards remains the same. However, the functions provided are highly configurable.
Meeting shrinking real-estate requirements
“Smart” I/O boards provide yet another important feature that minimizes system size and power: the ability to perform multiple functions on one PCB. In the past, many boards were available with 32 or 64 A/Ds or D/As or 28 V discretes, as well as separate boards for different types of communication functions (MIL-STD-1553, serial, ARINC 429, CANbus, and so on) and processing (SBCs). Many systems today, however, require smaller numbers of different types of I/O and communications.
To address this, manufacturers have created multifunction VME, CompactPCI, and VPX boards. These boards allow the system integrator to select from a large number of available functions and to incorporate smaller channel counts of many functions on one PCB. This is made possible through the baseboard FPGAs’ and DSPs’ ability to be programmed at final assembly to perform almost any task. For example, a single board can incorporate A/D, D/A, RTD, MIL-STD-1553, and ARINC 429, just to name a few.
Another possibility for real-estate savings is to use a multifunction I/O board that contains PowerPC- or Intel-based SBC support and the ability to provide large combinations of functions. A single board can replace up to six dedicated boards. Just as importantly, multifunction boards such as these can be COTS-based.
I/O data distribution and processing
The concept of I/O data distribution and processing is extremely powerful and can simultaneously solve a host of issues, such as reducing cable weight and mission computer processing requirements. The Sensor Interface Unit (SIU), shown in Figure 1, is usually some type of rugged, self-contained design that includes configurable I/O and provides communications with a mission processor. This approach allows system integrators to place I/O points very near the actual sensors, preprocess the data, and then send the reduced data back to the mission computer.
Figure 1: NAI’s Sensor Interface Unit
A small number of cables (usually just wire or fiber GbE) now needs to run from the SIU back to the main processor. One or more remote SIUs containing all of the I/O functions needed to perform this task, as well as an SBC function, could be used. The SIUs could be placed within a few feet (versus hundreds or even thousands of feet for systems with a single chassis) of the sensors and actuators to implement the control processing necessary to perform the actual function.
This situation presents significant weight reduction advantages: The weight difference between 100 twisted/shielded pairs, 50 feet in length versus a dual GbE wire or fiber connection of the same length is enormous – greater than a 10 to 1 reduction. Shorter cable lengths also result in reduced noise pickup, wire losses, and ground loop potentials, and in many cases reduced EMI. This can be most advantageous in aircraft, where hundreds of wires now need to travel only a few feet, versus 100 feet or more, or on ships and submarines that can be many hundreds of feet in length. Since such an SIU might use standard 28 VDC or single/three-phase 115 VAC, for example, the power cabling to the SIU is also short because this standard power is usually available across the entire platform at many points.
Redundancy is a good thing
Redundancy is another area where the SIU approach excels. Previously, redundant systems required entire racks of equipment to be duplicated, with the issue of which hardware and software decide which rack is in control, along with which processor. Usually, if anything failed in one rack, the entire rack needed to be shut down, and the backup took over. Distributed SIUs, however, can provide an inherent “fail-soft” property. Since each SIU controls a subset of all platform functions, if an SIU fails, only that subset needs to be switched over to the backup hardware. Additionally, each SIU likely has multiple GbE or other communications channels, so each is always connected to both the main and redundant mission computers. The main mission computer can stay in control of the system, despite various hardware failures. Figure 2 shows a conceptual redundant system design using this approach.
Figure 2: “FailSoft” system using SIUs and redundant mission processors
One manufacturer, North Atlantic Industries, offers an SIU that supports up to 300 I/O points – configurable for many different I/O functions – providing the equivalent of an SBC to process data and communicating to the main mission processor over a multitude of communication interfaces including GbE, MIL-STD-1553, ARINC 429, and CANbus, just to name a few.
Paul Feldman is V.P. of Digital Design Engineering at North Atlantic Industries. He has more than 30 years of experience in system, software, and electronic design. He received his BSEE and MSEE from the Polytechnic Institute of New York. He can be contacted at [email protected].
North Atlantic Industries 631-567-1100 www.naii.com